1. Field of the Invention
The present invention relates to a multilayer wiring board and, in particular, to a multilayer wiring board including base material layers and constraining layers that are alternately stacked.
2. Description of the Related Art
In existing technology, as illustrated by a cross-sectional view in FIG. 7, a multilayer wiring board 110 has been produced by stacking and pressing constraining layers 103a to 103d, each sandwiched between neighboring glass ceramic green sheet groups 102a to 102e, each including at least one layer. The glass ceramic green sheet groups 102a to 102e define basic material layers. The constraining layers 103a to 103d are made of an inorganic composition that is not sintered under a firing condition of the glass ceramic green sheet groups 102a to 102e. A conductor pattern defining one of internal electrodes 105a to 105d or one of external electrodes 107 and 108 is provided on a surface of one of the glass ceramic green sheet groups 102a to 102e and the constraining layers 103a to 103d. Through-holes that pass through the glass ceramic green sheet groups 102a to 102e are filled with conductive materials defining via-hole conductors 104a to 104e, respectively. Subsequently, the glass ceramic green sheet groups 102a to 102e are fired. In the multilayer wiring board 110, the constraining layers 103a to 103d negligibly contract during firing. Accordingly, contraction of the glass ceramic green sheet groups 102a to 102e in the surface direction is constrained by the constraining layers 103a to 103d. Consequently, separation and cracking occurring at the interfaces between the glass ceramic green sheets can be prevented (refer to, for example, Japanese Unexamined Patent Application Publication No. 2000-315864).
In the multilayer wiring board having such a structure, for example, as illustrated by an enlarged cross-sectional view of a main portion shown in FIG. 8, when a conductor pattern 116 is disposed on a surface of a constraining layer 120 or a basic material layer 130 and a wiring electrode 117 used for a required connection is disposed in a basic material layer 132 (in the middle of the basic material layer 132) located between constraining layers 120 and 122, and if one end of a via-hole conductor 114 is connected to the wiring electrode 117, cracking 115 may occur in the basic material layer 132 in the vicinity of an end 114x of the via-hole conductor 114, that is, in the vicinity of a connection portion between the via-hole conductor 114 and the wiring electrode 117.
The cracking 115 occurs because, although contraction of portions of the basic material layer 132 in contact with the constraining layers 120 and 122 in the surface direction is sufficiently prevented during a firing process, portions remote from the constraining layers 120 and 122 are not sufficiently constrained by the constraining layers 120 and 122. Accordingly, the portions remote from the constraining layers 120 and 122 also contract in the surface direction. That is, when the via-hole conductor 114 is disposed in the basic material layer 132 and if the via-hole conductor 114 extends to the middle of the basic material layer 132, a complicated contraction force that pulls the basic material layer 132 is exerted against a portion in the vicinity of the end 114x of the via-hole conductor 114. As a result, cracking that begins at the end 114x of the via-hole conductor 114 occurs in the vicinity of the wiring electrode 117. The cracking that occurs in the vicinity of the wiring electrode 117 decreases the reliability of wiring of the multilayer wiring board. If the wiring electrode 117 is not disposed in the basic layer in order to prevent the occurrence of cracking, the wiring efficiency of the multilayer wiring board decreases.